There have been so many small instances in my life where I have noticed this. Once a guy dropped into my apt and was going to leave for India the next day. I had briefly met him in a trip previously that lasted for 3 days. He had a strange personality and not very sociable or welcoming. I thought the trip was just a temporary thing and that he was going to return to US after the vacation. Later, I discovered that he was permanently going back to India. This revelation totally changed my thoughts and behavior towards him. I started giving him much more attention and spoke to him for long since I knew it was the last time I would ever see him.
The deeper I think the more I realize that there are a lot of things we see for the last time in our lives. Its like stepping twice in a river, the river is actually different every single time. There are so many people we meet just once or places that we may never see again. You may say there is a probability of seeing them again, but a LOT of times that is extremely low especially if your traveling around the world, studying abroad etc, or a lifestyle that involves a lot of changing environments.
Even the stuff we take as constant are actually not on close observation. My thoughts are continuously changing if I observe them keenly. Even my mental state of mind and physical sensations are unique at every moment. There is nothing to grasp, unless I put a huge amount of effort to maintain a status quo. For example: Say I regularly fantasize about sex related content, but sexual content is just a concept, the kind of feelings, quality of mental images and creativity is very different even if I do this twice everyday. Even this effort is just a trap that makes us do the same thing over and over again.